Origin8’s Sentinel: Mars Defense was a nice departure from cute graphics and light-hearted atmosphere among the bevy of tower defense games at the App Store (not to mention it was the first TD to successfully merge the Sci-Fi theme with the genre). It was dark and suspenseful, and planning for the bosses while defending the barriers from the hordes was challenging. Now with their latest sequel, Earth Defense, Origin8 looks to improve on an already impressive Sentinel with more weaponry and some new and welcomed features.

App Description: Sentinel 2: Earth Defense expands upon the acclaimed gameplay of Sentinel: Mars Defense. The action takes place across varied, lushly rendered Earth environments, and the alien assault force has grown more vicious than ever. Luckily, human turret technology has also advanced, and the ship weapons system provides a spectacular new way to eradicate invaders!

New in Sentinel 2:

Unleash the Sentinel’s incredible ship weapons system – if you’ve got the power to do so…

Full online high scores, achievements, friends and chat using OpenFeint

Exclusive new music written by Specimen A

Sentinel brought the concept of barriers to defend, the first I had seen that in the TD genre, as well as repair/harvesting drones to help repair damaged barriers and increase the cash flow. Now, Earth Defense introduces ship weapons that can help defend the barriers at key times. It does take a while for the weapons to recharge, and the longer you recharge, the more powerful weapons you get at your disposal.

From the get go, I did get mauled more than a few times trying that balancing act, but thankfully, I did get the hang of the strategy required. I have to admit that it took me a while to even figure out that there were ship weapons, which meant I got slaughtered halfway through the first map a few times over. After getting it, I still got slaughtered, as I didn’t save them for the bigger and nastier baddies. So in this follow up to the first, despite getting new weapons, it’s still a difficult balancing act of knowing just which towers to build and more importantly, when to use the ship weapons system.

The other challenge lies in maintaining the defenses while saving up for and buying repair drones and ship recharging drones. The extra money flow, extra ship energy, and barrier repairs become essential to your success during the later waves. Every map has a different challenge, but if you can get the most powerful bombs and lasers at keypoints, and save the flying laser drones for the big cheeses, then you’ll likely breeze through each level.

Some other features that come with Sentinel 2 include listening to your own music collection while playing the game and OpenFeint, the most popular iDevice social gaming platform that will allow for sharing of highscores and even chatting with friends. I was pretty satisfied with the game soundtrack, as it really does set the atmosphere for this TD, so I didn’t make use of the playlist feature often. As well, since I didn’t think any of my scores were worth sending online, seeing as I played the game on medium difficulty, I didn’t make full use of OpenFeint.

Earth Defense comes with four difficulty settings: easy, medium, hard, and psycho. Medium gave me a good enough drubbing a few times, I wasn’t about to try anything more difficult.

Overall Sentinel 2 presents new challenges with added parts to the gameplay, which makes it not just a rehash of the original, but with new strategies that are required to succeed. The ship weapons, while at first appearing to make the game too easy, are well integrated into the game. It’s a game that follows up, but unlike a lot of sequels, Origin8’s latest delivers a nice new variety of challenges while maintaining the same feel of Mars Defense.

Sentinel 2: Earth Defense is a definite recommendation for any TD enthusiast and if you’re a fan of the genre, I don’t think you can go wrong with this purchase.